Post-translational protein modification is a component of many cellular pathways. Prominent examples include methylation, phosphorylation, acetylation, and ubiquitylation. Post-translationally methylated lysine and arginine residues are central players in epigenetic pathways and are the subject of intense research into their roles in healthy development, stem cell pathways, and disease. DNA-packaging histones were the first proteins whose methylation was intensely studied, but it is increasingly clear that all plant and animal proteomes have many hundreds of methylated proteins.
Methylation stands apart from other modifications in multiple ways. It is the smallest group that can be added to a biomolecule. While many post-translational modifications (or “PTMs”) create dramatic changes in a protein's physico-chemical properties by installing charge on a neutral site (phosphorylation) or rendering a charged residue neutral (acetylation, citrullination), methylation does not significantly change the charge or pKa's of lysine or arginine side chains. Unlike all other PTMs, methyl groups are installed and removed by enzymes that must control the number of resulting methyl groups installed with high specificity. For example, lysine can be mono-, di-, or trimethylated, and arginine can be monomethylated or dimethylated (with dimethylarginine existing as two isomeric marks). Even when they occur at the same site, each kind of methyl mark encodes distinct epigenetic signaling outcomes. “Methylation” therefore defines an entire class of PTMs that generate enormous diversity in biochemical structure and function, without generating a large change in physico-chemical or stereoelectronic properties.
Analysis of methyl marks and other PTMs, however, remains difficult using conventional techniques. There exists a need in the art for sensitive chemical affinity based methods that can identify and separate post-translationally modified analytes.